In recent decades, vascular stenting has become an important therapy for treating occlusive vascular disease, including coronary artery disease, carotid artery disease and peripheral artery disease. A stent, also known as a vascular scaffold, is a tubular structure that is used, sometimes in conjunction with an angioplasty balloon catheter, to open up a stenosis or narrowing in a blood vessel and to hold the blood vessel open to allow improved blood flow. Stents are also used to treat strictures or narrowings in body passages other than blood vessels. Vascular stents are typically grouped into two general categories: balloon expendable stents and self-expanding stents. The bioresorbable vascular stent used in the present invention can be considered to be a hybrid of these two types. The bioresorbable vascular stent is heat treated to have a shape memory that makes the stent expand toward its deployed diameter. This behavior is temperature dependent. Above the glass transition temperature Tg of the stent material, the stent expands quickly, but at body temperature, the stent expands more slowly. An inflatable balloon is therefore used to accelerate the deployment of the stent, but even after deployment, the stent will continue to expand slightly, which assists in apposing the stent struts to the vessel wall. The stent could therefore be considered to be a balloon-assisted self-expanding stent or a self-apposing balloon expandable stent. For the crimping process however, the bioresorbable vascular stent can be treated much like a balloon expandable stent.
Typically, a balloon expandale stent is mounted on a stent delivery catheter by crimping (i.e. squeezing) the stent onto an inflatable balloon located near the distal end of the catheter.
Specialized crimping devices and automated machines have been devised for crimping stents onto balloon catheters. See, for example: U.S. Pat. No. 8,141,226; PCT International Application WO 2004/019768 and U.S. Patent Application 2002/0138966. Where allowed, these and all patents and patent applications referred to herein are hereby incorporated by reference. The stent crimping devices described in these patents can be modified for use with the crimping method of the present invention by adding controlled heating and cooling the crimp head.
Most balloon expandable stents used today are metal stents. However, there is an emerging field of bioabsobable or bioresorbable polymeric vascular stents. The terms bioabsorbable and bioresorbable are used interchangeably in the medical device industry to describe a material that, after implantation in the body, breaks down over time and is absorbed or resorbed by the surrounding tissues. Typical materials for bioabsorbable or bioresorbable stents include polylactic acid (PLA) and polyglycolic acid (PGA) polyglactin (PLAGA copolymer). Additional stent materials suitable for the present invention are described in U.S. Pat. No. 7,731,740 and PCT International Application WO 2005/096992. In general, a polymer with a glass transition temperature (Tg) of a least 45° C. is preferred.
Polymeric vascular stents present particular challenges in stent crimping. U.S. Pat. No. 7,743,481 describes an apparatus and method that are particularly adapted for crimping polymeric vascular stents. This stent crimping apparatus can be modified for use with the crimping method of the present invention by adding controlled heating and cooling.
Various methods have been devised for crimping balloon expandable stents that involve a step of inflating the balloon on the catheter during the crimping process. Examples of these methods are described in the following and patent applications: U.S. Pat. No. 5,836,965 (cf. FIG. 3); U.S. Pat. No. 5,976,181; and U.S. Pat. No. 8,123,793 (cf. FIG. 4).
Generally, the methods described in these patent references are not suitable for application to bioabsorbable or bioresorbable polymeric vascular stents. One fundamental difference between metallic stents and polymeric stents is that tubular metallic stents are typically fabricated at a diameter that is just slightly larger than their undeployed or crimped diameter. During the crimping step, the diameter of the stent only needs to be reduced by a small amount. Thus, when the balloon is inflated during the crimping process, it cannot assume its fully expanded diameter because it is constrained by the stent and the crimping apparatus. If the balloon were to be fully inflated it would impart irreversible plastic deformation to the stent struts, which would be highly deleterious to the stent. On the other hand, tubular polymeric stents are fabricated at a diameter that is close to their deployed of fully expanded diameter. During the crimping process, the diameter of the stent must be reduced from the deployed or fully expanded diameter to the undeployed or crimped diameter. Elevating the temperature of the polymeric stent at or above Tg during crimping avoids the problem of irreversible plastic deformation that occurs with metallic stents. Because it takes into account these differences, the crimping method of the present invention is particularly advantageous for crimping a bioabsorbable or bioresorbable polymeric vascular stent onto an inflatable balloon of the stent delivery catheter.